The envelop glycoprotein of HIV is critical for virus entry into cells and subsequent cytopathic effects. The transmembrane glycoprotein subunit (gp41) is comprised of distinct structural domains: 1) a membrane-spanning anchor domains; 2) an exterior gp41 domain critical for membrane fusion and the association with gp120; and 3) an intracytoplasmic domain whose functions are poorly understood. The strategies which are employed by lentiviruses to efficiently incorporate their viral glycoproteins during virion budding are not understood. However, the cytoplasmic domains of glycoproteins of some enveloped viruses have been shown to interact with viral core proteins during virus assembly and budding. Remarkably, the length of the intracytoplasmic domain of HIV (- 170 AA) is much grater than that of classical C-type retroviruses (-20AA). HIV-2 was previously restricted to West Africa but has now been identified in at least 37 nations worldwide (W.H.O, 1992). Interestingly, available evidence suggests that HIV-2 is less virulent than HIV-1. In addition, unique properties of the HIV-2 envelope glycoprotein have been reported including marked cytoplasmic domain truncations. The overall objectives of this proposal are: 1) To investigate the biological significance of the unique extended intracytoplasmic domain present in wild-type HIV; and 2) to further define the mechanisms by which the glycoproteins of HIV-2 and HIV-1 function in viral assembly, receptor binding, and membrane fusion. Our specific aims are: 1. To investigate potential mechanisms by which the intracytoplasmic domain modulates HIV-2 glycoprotein processing and fusion activity. We will express wild-type or mutant env genes and investigate glycoprotein transport, processing, and surface expression; gp41/32 association with gp120; CD4 affinity; and fusion-inhibitory properties of the cytoplasmic domain. 2. To determine the mechanism(s) for differences in CD4 affinity of the assembled glycoprotein complex compared with the soluble form of gp120. We will investigate whether this phenomenons established by (i) anchorage of gp120 by gp41/32 or (ii) the multimerization of gp120 molecules. 3. To investigate the mechanism for the selective inclusion of viral glycoproteins in budding HAV-2 particles. The role of specific glycoprotein signals for assembly into viral particles will be investigated with pseudotyping experiments, mutagenesis, and chemical cross-linking.